Node won't appear even when I search for it
I believe the AI/NN-based Denoise Node is new in Blender 2.81. Blender 2.81 is still in Beta / experimental development, so if you have the current stable version of Blender (2.80), you will not yet have the Denoise Node. You can download Blender 2.81 Beta under the experimental builds download section of the Blender page, if you'd like:

https://builder.blender.org/download/